Cap enforcement: refuse PRs at the cap in gh pr create and pmat work dispatch, drafts included #5518

Operator order (relayed 2026-10-18): enforce the open-PR cap in the tools. Refuse to open a PR at the cap in gh pr create (the wrapper/hook used here) and in pmat work dispatch, drafts included. Today the cap (FLOW-001) is advisory and the open count is 16-17 against 10, which is how W reached ~22 h (lambda_max = cap / W).
Scope: one contract (cap value read from one place, no second list), one check used by both entry points, a refusal message that names the count and the oldest open PR, and a clear exit code. Stale PRs count (STALE PR rule: clearing stale PRs comes before opening new ones). Raising the cap to 30 stays step 4 (#5508) and is sized from measured W.
Test first: at cap -1 allowed, at cap refused, a draft counts, an external-repo PR is excluded, a failed or rate-limited count read = not_measured (L23: refuse nothing on a failed read, say so). Reads go over GraphQL (GH-2), one call, ETag.
Falsifier: if opens at the cap still occur after this ships, a third entry point exists (web UI, API); list it.
Epic #5510 (FLOW-005 v2). One-way door: needs a planted quorum (C314) with lanes that are not the author model, then a non-author sign-off on the PR. Pmat ticket, test first, contract, PR <=200 lines via ci / gate; full tests stay on the merge-group commit. Questions go to quorum, not to the operator. No Python. LAB numbers never gate a PR (C324).